Step 1: Roast the Tomatoes and Garlic

1. Preheat your oven to 400°F (200°C). Preheating ensures the oven is at the correct temperature when you place the tomatoes in, allowing for even roasting and caramelization.
2. Prepare the tomatoes: Wash the Roma tomatoes and cut them in half lengthwise. This exposes more surface area for roasting and allows them to cook more quickly.
3. Arrange on a baking sheet: Place the halved tomatoes cut-side up on a large baking sheet. This orientation allows the juices to concentrate and the tomatoes to caramelize properly.
4. Add garlic: Scatter the unpeeled garlic cloves among the tomatoes on the baking sheet.
5. Drizzle with olive oil: Drizzle the tomatoes and garlic with olive oil, ensuring they are lightly coated.
6. Season: Sprinkle the tomatoes and garlic with dried oregano, dried thyme, red pepper flakes (if using), salt, and freshly ground black pepper. Toss gently to combine and ensure even seasoning.
7. Roast: Place the baking sheet in the preheated oven and roast for 40-45 minutes, or until the tomatoes are softened, slightly caramelized, and have released their juices. The roasting time may vary slightly depending on your oven and the ripeness of the tomatoes. They should be tender and slightly wrinkled.
8. Cool slightly: Remove the baking sheet from the oven and let the tomatoes and garlic cool slightly. This will make them easier to handle in the next step.

Step 2: Prepare the Roasted Tomato Sauce

1. Squeeze roasted garlic: Once the garlic cloves are cool enough to handle, squeeze the roasted garlic pulp out of their skins into a bowl. Discard the skins. Roasted garlic should be soft and easily squeezed out.
2. Combine roasted tomatoes and garlic: Transfer the roasted tomatoes and their juices from the baking sheet to the bowl with the roasted garlic.
3. Blend (optional): For a smoother sauce, you can use an immersion blender or transfer the mixture to a regular blender and blend until smooth. Alternatively, you can leave the tomatoes chunky for a more rustic texture. If using a regular blender, be careful when blending hot liquids and vent the lid to prevent pressure buildup.
4. Set aside: Set the roasted tomato sauce aside while you prepare the macaroni and cheese sauce.

Step 3: Cook the Macaroni

1. Cook macaroni according to package directions: While the tomatoes are roasting, cook the elbow macaroni according to package directions for al dente. Al dente pasta will hold its shape better in the baked mac and cheese and prevent it from becoming mushy.
2. Drain macaroni: Once cooked, drain the macaroni well and set aside. It’s important to drain the pasta thoroughly to prevent excess water from diluting the cheese sauce.

Step 4: Make the Cheese Sauce

1. Melt butter in a large saucepan: In a large, heavy-bottomed saucepan over medium heat, melt the butter. A heavy-bottomed saucepan will distribute heat evenly and prevent scorching.
2. Add flour and cook roux: Once the butter is melted, add the all-purpose flour and whisk continuously for 1-2 minutes until the roux is smooth and lightly golden. This cooks out the raw flour taste and creates a base for the cheese sauce. Don’t let the roux brown too much, as it will affect the color and flavor of the sauce.
3. Gradually whisk in milk and cream: Slowly whisk in the whole milk and heavy cream, a little at a time, ensuring each addition is fully incorporated before adding more. This prevents lumps from forming and creates a smooth sauce. Continue whisking until the sauce is smooth and begins to thicken slightly.
4. Simmer and thicken: Bring the sauce to a gentle simmer over medium-low heat and cook, stirring occasionally, for 5-7 minutes, or until the sauce has thickened enough to coat the back of a spoon. Simmering allows the sauce to thicken and the flavors to meld together.
5. Remove from heat and add cheese: Remove the saucepan from the heat. Gradually add the grated sharp cheddar cheese and Gruyere cheese, stirring constantly until the cheese is completely melted and the sauce is smooth and creamy. Adding the cheese off the heat prevents it from becoming grainy or oily.
6. Stir in mustard and nutmeg: Stir in the Dijon mustard and nutmeg. These ingredients add depth and complexity to the cheese sauce.
7. Season to taste: Season the cheese sauce with salt and freshly ground black pepper to taste. Remember to taste and adjust seasoning as needed.

Step 5: Combine and Bake

1. Preheat oven to 375°F (190°C). Preheating the oven again for baking ensures the mac and cheese bakes evenly and develops a golden crust.
2. Combine macaroni, cheese sauce, and roasted tomato sauce: Add the cooked and drained macaroni and the prepared roasted tomato sauce to the cheese sauce in the saucepan. Stir gently to combine everything evenly, ensuring the macaroni is well coated with both sauces.
3. Transfer to baking dish: Pour the macaroni and cheese mixture into a 9×13 inch baking dish or a similar sized oven-safe dish. A baking dish of this size allows for even baking and a good proportion of crispy topping if using breadcrumbs.
4. Add breadcrumb topping (optional): If using breadcrumbs, sprinkle them evenly over the top of the macaroni and cheese. For extra flavor, you can toss the breadcrumbs with melted butter and grated Parmesan cheese before sprinkling.
5. Bake: Bake in the preheated oven for 20-25 minutes, or until the macaroni and cheese is heated through, bubbly around the edges, and the topping (if used) is golden brown. Baking time may vary slightly depending on your oven.
6. Rest and serve: Remove from the oven and let the Roasted Tomato Macaroni and Cheese rest for 5-10 minutes before serving. This allows the sauce to set slightly and makes it easier to serve. Garnish with fresh parsley, if desired, and serve hot.